Paleoproterozoic ultrahigh-temperature pelitic granulites in the northern Sulu orogen: Constraints from petrology and geochronology

Abstract

Ultrahigh-temperature (UHT) pelitic granulites are reported firstly in Weihai area, the northern segment of Sulu ultrahigh-pressure (UHP) metamorphic belt. The pelitic granulites, together with other metamorphic supracrustal rocks, including gneisses, marbles, quarzites and calc-silicates, occur as blocks within the orthogneisses which formed at Neoproterozoic but underwent Triassic UHP metamorphism. The pelitic granulites have a peak mineral assemblage of garnet+sillimanite+K-feldspar+plagioclase (antiperthite)+quartz+rutile+ilmenite±biotite. The P–T pseudosection and mineral isopleths thermobarometry modeled in the MnNCFKMASHTO system constrain P–T conditions of 11–13kbar and >900°C for the peak stage. In situ LA–ICPMS U–Pb dating and trace element analyses show that zircons from the two granulites have low Th/U ratios, flat HREE patterns with negative Eu anomalies, typical of granulite-facies metamorphic zircon. These zircons yielded 207Pb/206Pb ages of 1818±10Ma to 1854±9Ma, representing the time of the UHT granulite-facies metamorphism. Our result, together with previous investigations, indicates that the Mid-Paleoproterozoic tectonothermal event was widespread in the northern margin of the South China Craton. The Paleoproterozoic UHT granulite-facies metamorphism and associated anatexis, and coeval magmatism possibly derived from accretionary orogeny during the assembly of the supercontinent Columbia.